1. Field of the Invention
The present invention relates to a light scanning device which performs light scanning, a production method therefor, and an image forming apparatus such as a copying machine, a printer, a facsimile, and a plotter in which the light scanning device is incorporated.
2. Description of the Related Art
For example, Japanese Patent Application Publication (JP-B) No. 7-19084 (Patent Document 1) discloses a tandem-type image recording apparatus in which plural recording devices are arranged, and images are superposed and transferred while a transfer paper is sequentially conveyed to the recording devices by a transfer belt. Each of the recording devices includes: a laser beam exposure unit which converts image light obtained by color separation of recording information into a digital signal and projects the digital signal onto a photosensitive member; a development unit which develops an electrostatic latent image on the photosensitive member; and a transfer unit which transfers a visualized image on the photosensitive member to transfer paper. In the tandem-type image recording apparatus, a measuring pattern image is formed on a conveyance belt in each color to detect passage of each color pattern image, a difference between detection timing and a setting value is computed from a detection signal for each color, and write start timing is adjusted to realize a decrease in shift between colors and improvement of image quality by a write start timing signal which is arbitrarily changeable in response to the value from the computing unit.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2006-35623 (Patent Document 2) discloses a light scanning device including: a color shift detection unit which detects position shift between visualized images corresponding to colors; a laser beam detector which detects a scanning position of a laser beam; and a scanning position changing unit which changes the scanning position of the laser beam in a sub-scanning direction. The laser beam detector outputs a change signal for changing the scanning position of the laser beam to the scanning position changing unit according to the detected color shift amount, and detects again the scanning position of the laser beam corresponding to each color before image formation. The scanning position changing unit changes the scanning position again without performing the image forming process when the detection result performed again by the laser beam detector is not lower than a setting value.
The light scanning device is characterized in that a time necessary for the change performed by the scanning position changing unit is shorter than a non-image formation time between pages during continuous print.
However, in the technique disclosed in Patent Document 1, even if the color shift is generated by temperature rise with time during use of the image forming apparatus, the image deterioration with time cannot be prevented because the image forming apparatus has no color shift correction unit.
In the technique disclosed in Patent Document 2, it is necessary to extremely shorten the time for changing the scanning position of the laser beam, and it is also necessary that a driving unit for rotating an optical element which is of the changing unit be driven at high speed. Therefore, there are problems such as an increase in power consumption, deformation of the optical element caused by the temperature rise, a decrease in image quality caused by deterioration of a rotating mechanism unit, and inhibition of stabilization of image quality.
Some of color image forming apparatus such as a color laser printer include plural scanning imaging optical systems. In the color image forming apparatus, the plural light scanning units perform the laser beam scanning to independently write information on plural different colors onto plural photosensitive members rotated by a driving mechanism, whereby plural electrostatic latent images are formed. Then, the plural electrostatic latent images are visualized in plural visualizing units, and the visualized images are superposed on a transfer material to form a color image.
Specifically, in each of the light scanning units, a semiconductor laser driven according to an image information signal of each color emits the laser beam. The laser beam is collected onto a surface of a uniformly-charged photosensitive member through optical components such as a polygon mirror and a lens, and is scanned in a main scanning direction.
The electrostatic latent image is formed by writing an image signal into the rotating photosensitive member surface according to the plural scanning beams having predetermined intervals.
In the color image forming apparatus, a time from the latent image formation to the transfer and a distance between photosensitive members are different depending on eccentricity of the photosensitive member and a variation in diameter. In addition, a registration position shift of each toner image in the sub-scanning direction is generated by a fluctuation in speed and meandering of the transfer body, the transfer belt, or a conveyance belt for conveying recording paper, which generates the color shift to deteriorate the image quality.
Additionally, in the light scanning device, unless the write start timing of an electrostatic latent image formed in the photosensitive member is correctly adjusted in each color, the color shift is generated due to the registration position shift.
Conventionally, the registration position shift is periodically detected during start-up of the apparatus or between jobs by a registration position shift detection pattern recorded in the transfer body, and the registration position is corrected in the sub-scanning direction by measuring the write start timing every two polygon mirror surfaces.
When the laser beam traveling toward the photosensitive member is set to pass through different optical paths because of restriction of component arrangement, the scanning position fluctuates easily by ambient temperature at which the color image forming apparatus is installed. The scanning position shift is periodically detected and corrected during start-up of the apparatus or between jobs by the registration position shift detection pattern recorded in the transfer body. However, in this case, the scanning position further fluctuates by heat generation of a fixing device and a polygon mirror motor associated with a continuous printing operation. Therefore, when the number of prints is increased in one job, there is also generated a problem in that the color shift is gradually increased.